As feature sizes and pitches shrink in the fabrication of integrated circuit devices, the resolution of the projection optics in conventional optical lithography systems begins to limit the quality of photoresist images created on wafers using binary photomasks that only modulate the intensity of the projected electromagnetic radiation. Specifically, electromagnetic radiation from a light source of the optical lithography system is completely blocked by the opaque features in the binary mask and is fully transmitted through transparent mask regions. Alternating phase shift masks (altPSM's) represent a powerful resolution enhancement technique (RET) that can effectively increase the resolution of conventional optical lithography systems by introducing a 180° or half-wavelength phase shift in the light transmitted between neighboring windows of the mask. This phase shifting is accomplished by creating a path length difference for the exposing electromagnetic radiation in the mask material between adjacent windows on the phase shift mask. Features are formed in exposed resist by the diffraction edges due to destructive interference. Hence, altPSM's not only modulate the intensity of the transmitted electromagnetic radiation, but its phase as well.
The optical path length difference is created by recessing phase shift windows in a blank of the transparent mask material to a depth proportional to the source wavelength and inversely proportional to the refractive index of the mask material. The mask blank is typically fabricated by patterning a layer of an opaque material deposited on the mask blank and then providing recessed phase shift windows in the unmasked areas of the patterned opaque layer by reactive ion etching (RIE). The fabrication of altPSM's is limited by the uniformity of the etch process. Specifically, the mask blank lacks an etch stop layer so that the inherent pattern factor and across-wafer substrate spatial variations of the etch rate in the RIE process chamber are reproduced in the mask blank. The variations in etch depth arising from the etch rate variations degrade depth of focus and effectively reduce resolution in the finished altPSM as the phase shift windows have different depths in different regions of the altPSM.
What is needed, therefore, is a method of fabricating an alternating phase shift mask that forms phase shift windows without directly etching a mask blank.